You’re not using a meat thermometer
No one uses meat thermometers, which is why few people pull together steakhouse quality meat. That applies to burgers, too. You just can’t eye-ball it; you need the meat to cook through if you want juicy and flavorful patties. You’re aiming at 160 degrees Fahrenheit for a burger pattie’s internal temperature.
You don’t have enough space
You need enough counter space to make proper burgers; that applies to every type of cooking, but it’s especially hard when at an outdoor dinner party. Being organized is vital. You must have all your components, from the meat to the buns and from your dressings to the toppings in hand. Make sure you have a side table close by.